Unusually for a lego game, the red bricks are really easy to obtain. Mostly using Joker or Riddler so as soon as you can get them your sorted.

I would also get Lex Luther asap because he will help obtain the mini kits and gold bricks.

Tips, complete full play through (i wouldn't even mess around collecting stuff, as you know you have to play all lego games twice anyhow lol) once you have completed the story, get gold bricks from the open map via the bat cave (Superman is your best friend for this) then start unlocking your characters by building the gold lego doors. Before you know it, your at 50%